A phased array refers to an antenna having a large number of radiating elements that emit phased signals to form a radio beam. The radio signal can be electronically steered by the active manipulation of the relative phasing of the individual antenna elements. The electronic beam steering concept applies to antennas used with both a transmitter and a receiver. Electronically scanned phased array antennas are advantageous in comparison to their mechanical counterparts with respect to speed, accuracy, and reliability. The replacement of gimbals in mechanically scanned antennas with electronic phase shifters in electronically scanned antennas increases the survivability of antennas used in defense systems through more rapid and accurate target identification. Complex tracking exercises can also be maneuvered rapidly and accurately with a phased array antenna system.
Phase shifters play key role in operation of phased array antennas. Electrically controlled phase shifters can utilize tunable ferroelectric materials, whose permittivity (more commonly called dielectric constant) can be varied by varying the strength of an electric field to which the materials are subjected. Even though these materials work in their paraelectric phase above the Curie temperature, they are conveniently called “ferroelectric” because they exhibit spontaneous polarization at temperatures below the Curie temperature. Tunable ferroelectric materials including Barium Strontium Ttitanate (BST) or BST composites have been the subject of several patents.
Dielectric materials including barium strontium titanate are disclosed in U.S. Pat. No. 5,312,790 to Sengupta, et al. entitled “Ceramic Ferroelectric Material”; U.S. Pat. No. 5,427,988 to Sengupta, et al. entitled “Ceramic Ferroelectric Composite Material-BSTO-MgO”; U.S. Pat. No. 5,486,491 to Sengupta, et al. entitled “Ceramic Ferroelectric Composite Material - BSTO-ZrO2”; U.S. Pat. No. 5,635,434 to Sengupta, et al. entitled “Ceramic Ferroelectric Composite Material—BSTO-13 Magnesium Based Compound”; U.S. Pat. No. 5,830,591 to Sengupta, et al. entitled “Multilayered Ferroelectric Composite Waveguides”; U.S. Pat. No. 5,846,893 to Sengupta, et al. entitled “Thin Film Ferroelectric Composites and Method of Making”; U.S. Pat. No. 5,766,697 to Sengupta, et al. entitled “Method of Making Thin Film Composites”; U.S. Pat. No. 5,693,429 to Sengupta, et al. entitled “Electronically Graded Multilayer Ferroelectric Composites”; and U.S. Pat. No. 5,635,433 to Sengupta, entitled “Ceramic Ferroelectric Composite Material-BSTO-ZnO”. These patents are hereby incorporated by reference. A copending, commonly assigned U.S. patent application titled “Electronically Tunable Ceramic Materials Including Tunable Dielectric And Metal Silicate Phases”, by Sengupta, filed Jun. 15, 2000, discloses additional tunable dielectric materials and is also incorporated by reference. The materials shown in these patents, especially BSTO—MgO composites, show low dielectric loss and high tunability. Tunability is defined as the fractional change in the dielectric constant with applied voltage.
Tunable phase shifters using ferroelectric materials are disclosed in U.S. Pat. No. 5,307,033, 5,032,805, and 5,561,407. These phase shifters include a ferroelectric substrate as the phase modulating elements. The permittivity of the ferroelectric substrate can be changed by varying the strength of an electric field applied to the substrate. Tuning of the permittivity of the substrate results in phase shifting when an RF signal passes through the phase shifter. The ferroelectric phase shifters disclosed in those patents suffer high conductor losses, high modes, DC bias, and impedance matching problems at K (18 to 27 GHz) and Ka (27 to 40 GHz) bands.
One known type of phase shifter is the microstrip line phase shifter. Examples of microstrip line phase shifters utilizing tunable dielectric materials are shown in U.S. Pat. Nos. 5,212,463; 5,451,567 and 5,479,139. These patents disclose microstrip lines loaded with a voltage tunable ferroelectric material to change the velocity of propagation of a guided electromagnetic wave. U.S. Pat. No. 5,561,407 discloses a microstrip voltage-tuned phase shifter made from bulk ceramic. Bulk microstrip phase shifters suffer from the need for higher bias voltage, complex fabrication processing and high cost. Coplanar waveguides can also serve as phase shifters. U.S. Pat. Nos. 5,472,935 and 6,078,827 disclose coplanar waveguides in which conductors of high temperature superconducting material are mounted on a tunable dielectric material. The use of such devices requires cooling to a relatively low temperature. In addition, U.S. Pat. Nos. 5,472,935 and 6,078,827 teach the use of tunable films of SrTiO3, or (Ba, Sr)TiO3 with high a ratio of Sr. SrTiO3, and (Ba, Sr)TiO3 have high dielectric constants, which results in low characteristic impedance. This makes it necessary to transform the low impedance phase shifters to the commonly used 50-ohm impedance.
U.S. Pat. No. 5,617,103 discloses a ferroelectric phase shifting antenna array that utilizes ferroelectric phase shifting components. The antennas disclosed in that patent utilize a structure in which a ferroelectric phase shifter is integrated on a single substrate with plural patch antennas. Additional examples of phased array antennas that employ electronic phase shifters can be found in U.S. Pat. Nos. 5,079,557; 5,218,358; 5,557,286; 5,589,845; 5,917,455; and 5,940,030.
It would be desirable to have a phased array antenna, which utilizes low cost phase shifters that can operate at room temperature and at high frequencies, such as above Ku band (12 to 18 GHz). This could play an important role in helping to make electronically scanned phased array antennas practical for commercial applications.